Power transmission mechanism

ABSTRACT

A shaft member  1  and an outer peripheral member  2  disposed on the outer periphery of the shaft member  1  are joined together by splines  3  through which the teeth  31  of the shaft member  1  and the teeth  32  of the outer peripheral member  2  are fitted together. At one axial end of each spline  3,  a trough  31  in the tooth  31  of the shaft member  1  is diametrically increased to provide a diametrically increased region S 1.  In this diametrically increased region S 1 , there is defined a fit region F for the tooth  31  of the shaft member  1  and the tooth  32  of the outer peripheral member  2.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a power transmission mechanismfor transmission of torque between two members.

[0002] Transmission shafts for transmission of power (torque orrotation) are used in many machine parts in automobiles and industrialmachines. Some shafts are solid and others hollow, these being producedby direct cutting or plastic working of bar or pipe material or, inrecent years, by the sintering of powder.

[0003] Spline shafts or serrated shafts for transmission of high torqueare generally formed by subjecting medium carbon steel or low alloysteel (case hardening steel, nitrided steel or the like) to a heattreatment, such as surface hardening process or tempering, for example,carburization hardening, high frequency hardening or nitriding, so as toincrease the shaft strength in consideration of plastic workability,machinability and cost, it being only after such treatment that theseshafts are put to use. Further, recently, use has been made ofnon-refine steel to dispense with refining, or a material subjected tohigh alloying or high purifying (reduction of inclusions, reduction ofP, etc.) to increase strength, or a material subjected to shot peeningto increase fatigue strength.

[0004]FIG. 5 shows an example of a machine part having said transmissionshaft, which is a constant velocity joint used in the drive shaft of anautomobile. This constant velocity joint includes a shaft member 11having an inner ring 12 fitted thereon through splines 13 formed on theouter periphery of said shaft member 11, the torque in the shaft member11 being transmitted to the inner ring 12 through the groove-and-ridgefit of the splines 13.

[0005] In this connection, there are various types as to the shaft ofthe spline terminal end side (C in the figure) of the shaft member11—the “terminal end side” means the opposite side when the shaft endsurface which, when the shaft member 11 is inserted in the inner ring12, is the first to fit in the inner ring 12 is taken as the inlet side.FIGS. 6 through 9 show examples thereof, FIG. 6 showing a first type(tentatively referred to as the “cut-through type”) in which the splinetrough 11 a of the spline 13 is directly cut through the outerperipheral surface of the shaft member 11, FIGS. 7 through 9 showing asecond type (tentatively referred to as the “cut-up type”) in which thespline trough 11 a is smoothly diametrically increased until it connectsto the outer peripheral surface of the shaft member 11. Different formsof the cut-up type are known: one in which the diametrical increase iseffected by an arc with a radius R1 (FIG. 7), one in which thediametrical increase is effected by an arc with a greater radius R2 thanin FIG. 7 (R2>R1) (see FIG. 8), and one in which the diametricalincrease takes a spherical form with a radius SR (see FIG. 9).

[0006]FIG. 10 shows a conventional fit between said shaft member 11 andinner ring 12, wherein a relief region T′ where the inner diameter isincreased is defined in the inner ring 12 on the terminal end side ofthe spline ridge 12 b, and the portion of the ridge 12 b excluding therelief region T′ is fitted in the portion of the trough 11 a excludingthe diametrically increased region S′ of the shaft member 11, it beingarranged that such fitting portion F′ (marked with dots) does not enterthe diametrically increased region S′ of the trough 11 a of the shaftmember 11.

[0007] In recent years, with the global environmental problem beinghighlighted, it has been required in the automobile industry to tightenemission control and improve fuel efficiency, and as a measure therefor,lightening has been promoted. In automobiles, splines and serrations(hereinafter represented by the term spline shaft) have been usedabundantly in such parts as transmissions, differentials, drive shafts,and propeller shafts, it being noted that since the reduction of theweight of the spline shaft contributes much to the lightening of theautomobile, there has been a strong need to increase the spline shaftstrength, i.e., to increase the strength in two aspect: static strengthand fatigue strength.

[0008] As for the measures for strengthening and lightening the splineshaft, the aforesaid high alloying or high purifying may becontemplated, but these would not be advantageous from the viewpoint ofproduction cost since they are attended by an increase in the cost ofmaterial or a large decrease in workability. Further, shot peening,though effective in improving the fatigue strength, is not observed toprovide sufficient merits as to static strength; rather, it leads tohigh cost.

[0009] A spline shaft whose terminal end is increased in diameter in alarge arc form (FIG. 8) or in a spherical form (FIG. 9), though improvedin static strength as compared with the type shown in FIG. 7, is notobserved to provide sufficient merits in increasing the fatiguestrength, as can be seen from the test results shown in FIG. 13.Further, since working tools (hob cutters, forging racks, etc.) have tobe newly produced, a cost increase is incurred. On the other hand, thecut-through type shown in FIG. 6 is not suitable for weight reductionmeasures, since it is inferior to the cut-up type shown in FIG. 7 inboth static strength and fatigue strength, as is clear also from theexperimental results shown in FIG. 12.

[0010] As described above, the conventional measures for weightreduction are confronted with problems in either cost or strength andthere has been no measure that has successfully satisfied both of therequirements at one time.

[0011] Accordingly, an object of the present invention is to make itpossible to achieve improvements in the static strength and fatiguestrength of a spline shaft or serrated shaft without incurring anincrease in costs.

SUMMARY OF THE INVENTION

[0012] The boss of the inner ring was fitted on a spline shaft of thetype shown in FIG. 7 whose troughs were diametrically increased by anarc (for the specifications of the spline shaft, see FIG. 14), and thisassembly was put to torsion test to examine and analyze the fracturemode. As a result, it was found that as shown in FIG. 11, the fracturecomprised two main fracture planes A and B: a first fracture planeextending along the bottom of the trough 11 a of the shaft member 11 (A:axial fracture plane), and a second fracture plane inclined at an angleof 45° with respect to the axis (B: main stress fracture plane). It isbelieved that the axial fracture plane A is a shear fracture plane dueto an axially-acting shearing force and that the main fracture plane Bis a tensile fracture plane due to torsional main stress.

[0013] Next, the fit position of the boss was axially stepwise shiftedand the strength of the spline shaft was measured at each shiftedposition; the results shown in FIG. 15(A) were obtained. The horizontalaxis in this figure represents the position X [mm] at which the boss isfitted, and the left-hand vertical axis represents a ratio Y1 ofrepetition till the occurrence of fatigue fracture (the load shearstress is set to ±665 MPa [67.8 kgf/mm²] and the right-hand verticalaxis represents the rate of increase Y2 [%] in torsional break strength.The X on the horizontal axis, as shown in (B) of the same figure,indicates the distance from the terminal end 11a1 of the trough 11 a ofthe shaft member 11 to the point (æ) where the terminal end outline 12b1of the ridge 12 b of the boss 12 crosses the outer periphery level L ofthe shaft member 11. Measurements were conducted at positions X=a, b, .. . e, the repetition ratio Y1 and rate of increase Y2 being determinedwith the position a (X=6 mm) used as a reference (Y1=1, Y2=0). Further,(2), (4), (6), (10), and (12) in FIG. 15(A) indicate axial shear cracklengths [mm].

[0014] It is seen from FIG. 15(A) that the more the fit position of theboss is shifted toward the terminal end (left-hand side of the figure)of the shaft member 11, the more the axial shear fracture plane (axialshear crack) decreases, thus increasing the strength. The reason, as itis believed, is that the portion of the spline (non-fit portion) whichis not fitted to the boss during the torsion test is locally twisted andthat if the length of the non-fit portion is decreased, the local twistdecreases and so does the shear stress acting on the trough of the shaftin the non-fit portion.

[0015] It is seen from FIG. 15(A) that the static strength and fatiguestrength sharply increase after the boss has reached a particularfit-position which is immediately in front of the terminal end of thespline shaft. The critical position where the fatigue strength sharplyincreases lies in a region between the points b and c in (B) of the samefigure, said region roughly coincides with the position where theterminal end rise portion 12b1 of the ridge 12 b of the boss starts tocross the diametrically increased region S′ of the trough 11 a of theshaft member 11 (the position where the rise portion 12b1 of the ridge12 b starts to fit in the trough 11 a in the diametrically increasedregion S′).

[0016] It is believed that the reason is that in addition to thedecrease in the shear stress in the tooth bottom of the non-fit regiondescribed above, in the diametrically increased region S′ the trough 11a is diametrically increased, resulting in the tooth bottom beingdiametrically increased, so that the stress in this portion isdecreased.

[0017] The present invention, which is based on the view describedabove, is intended to provide a power transmission mechanism wherein ashaft member and an outer peripheral member disposed on the outerperiphery of said shaft member are connected for mutual torquetransmission by a fit between the teeth of the shaft member and theteeth of the outer peripheral member and the troughs of the teeth of theshaft member are increased in diameter at least at one axial end, saidpower transmission mechanism being characterized in that in thediametrically increased regions of said troughs, there are fit regionsfor the teeth of the shaft member and the teeth of the outer peripheralmember. In this case, the shaft member and the outer peripheral memberare connected by splines or serrations.

[0018] In the diametrically increased region of said trough, if thetrough in the tooth of the shaft member is kept in contact with theridge of the tooth of the outer peripheral member, then the fit regionis provided with a sufficient area to greatly increase the strength. Inthis case, if an arcuate chamfer is provided on the ridge of the toothof the outer peripheral member in contact with the trough in the toothof the shaft member, it is possible to bring the two in surface contactwith each other in the diametrically increased region, and the increasedarea of the fit region provides a further increase in strength.

[0019] Further, the ridges of the teeth of the outer peripheral membermay be brought into contact with the large diameter end (the terminalend) of the diametrically increased region of said trough, therebyproviding a sufficient area for the fit region, achieving a largeincrease in the shaft strength. In this case, it is preferable toprovide a restricting means for preventing the outer peripheral memberfrom moving toward the other axial end to serve as a means forpreventing the rattling of the outer peripheral member.

[0020] Such restricting means may be composed of a forcing means forforcing the outer peripheral member toward one axial end or a pressingmeans by which the teeth of the shaft member and the teeth of the outerperipheral member are circumferentially pressed against each other.

[0021] As described above, according to the invention, it is possible togreatly increase the static strength and fatigue strength of a splineshaft or serrated shaft. Furthermore, there are no such drawbacks asdecreased workability and increase in costs, as in the case of usinghigh alloy steel or highly purified steel, nor is there a markedincrease in facility introducing cost as in the case of shot peening.Thus, reduction in the cost and weight of the spline shaft is madepossible and in the present invention it is possible to achieve, forexample, a 12% reduction in weight since a 19% increase in strength canbe achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is an axial section of a power transmission mechanismaccording to the invention;

[0023]FIG. 2 is an axial section showing another embodiment of theinvention;

[0024]FIG. 3 is an axial section showing another embodiment of theinvention;

[0025]FIG. 4(A) is a table showing the effect of the surface hardness onthe torsional strength and fatigue strength, and (B) is a table showingexperimental data about the effect of the hardening depth (hardeningratio) on the torsional strength and fatigue strength;

[0026]FIG. 5 is an axial section of a constant velocity joint having apower transmission mechanism;

[0027]FIG. 6 is an axial section showing an example of the shape of theterminal end (C in FIG. 5) of a spline shaft;

[0028]FIG. 7 is an axial section showing an example of the shape of theterminal end (C in FIG. 5) of a spline shaft;

[0029]FIG. 8 is an axial section showing an example of the shape of theterminal end (C in FIG. 5) of a spline shaft;

[0030]FIG. 9 is an axial section showing an example of the shape of theterminal end (C in FIG. 5) of a spline shaft;

[0031]FIG. 10 is an axial section of a conventional power transmissionmechanism;

[0032]FIG. 11 is a section showing the torsional fracture mode of aspline shaft;

[0033]FIG. 12 is a table showing strength comparison data on the splineshafts shown in FIGS. 6 and 7;

[0034]FIG. 13 is a table showing strength comparison data on the typesshown in FIGS. 7, 8 and 9;

[0035]FIG. 14 is a table showing the specifications of samples used totest a spline shaft for torsional strength; and

[0036]FIG. 15(A) is a graph showing the results of torsional strengthtests, and FIG. 15(B) is an enlarged section of a power transmissionmechanism for explaining FIG. 15(A);

[0037]FIG. 16 is an axial section showing another embodiment of theinvention;

[0038]FIG. 17 is an axial section showing another embodiment of theinvention;

[0039]FIG. 18 is an axial section showing the fixing construction of anouter peripheral member;

[0040]FIG. 19 is an axial section showing a restricting means;

[0041]FIG. 20 is an axial section showing a restricting means;

[0042]FIG. 21 is an axial section showing a restricting means; and

[0043]FIG. 22 is a circumferential section showing a restricting means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] An embodiment of the invention will now be described withreference to FIGS. 1 through 22.

[0045] As shown in FIG. 1, a power transmission mechanism according tothe present invention comprises a shaft member 1 and an outer peripheralmember 2, such as a boss, disposed on the outer periphery of the shaftmember 1, said members being joined together by splines 3 (orserrations) through which the teeth 31 of the shaft member 1 and theteeth 32 of the outer peripheral member 2 are fitted together. The teeth31 and 32 extend axially.

[0046] Of a trough 31 a and a ridge 31 b in a tooth 31 formed on theouter periphery of the shaft member 1, the terminal end (right-hand sidein FIG. 1) of the trough 31 a has a diametrically increased region S1which is smoothly diametrically increased by an arc, and at the terminalend of said diametrically increased region S1, the trough 31 a connectsto the outer peripheral surface of the plain portion 1 a (formedadjacent the terminal end of the serration 3) of the shaft member 1. Onthe other hand, the terminal end of the ridge 31 b of the shaft member 1is slightly diametrically decreased and connects to the outer peripheralsurface of the plain portion 1 a of the shaft member 1 at the same placeas the terminal end 31a1 of the trough 31 a. From the viewpoint of thestrength of the shaft member 1, it is desirable that the outer diameterof the plain portion 1 a be equal or approximately equal to the outerdiameter of the ridge 31 b of the shaft member 1. The trough 31 a in thediametrically increased region S1, which is formed by an arc alone, maybe formed by a combination of an arc and a straight line (the arc beingdisposed at the inlet side), as shown in FIG. 2.

[0047] The outer peripheral member 2 is positioned relative to the shaftmember 1 in that the inner diametrical end of the terminal side abutsagainst a shoulder 1 b formed on the outer periphery of the shaft member1 and in that the inner diametrical end of the inlet side (left-handside in the figure) is locked by a stop ring (14: see FIG. 5). Of atrough 32 a and a ridge 32 b in a tooth 32 (indicated by hatching)formed on the inner periphery of the outer peripheral member 2, thetrough 32 a is formed straight to the terminal end of the outerperipheral member 2 with the same diameter maintained. On the otherhand, the terminal end of the ridge 32 b leads to a relief region Thaving a greater inner diameter than at the inlet side via an inclinedrise portion 32b1. The inner diameter of the relief region T is greaterthan the outer diameter of the plain portion 1 a of the shaft member 1but smaller than the outer diameter of a shoulder portion 1 b of theshaft member 1.

[0048] In the present invention, the fit region F (marked with dots) forthe tooth 31 of the shaft member 1 and the tooth 32 of the outerperipheral member 2 is defined not only in the spline effective regionS2 (which refers to the trough excluding the diametrically increasedregion S1) but also in the diametrically increased region S1, so as toallow both teeth 31 and 32 to circumferentially contact each other inthe diametrically increased region S1. This is sharply contrasted withthe conventional article shown in FIG. 10 in which the fit region F′ isdefined excluding the diametrically increased region S′ of the trough 31a.

[0049] Such arrangement makes it possible to increase the staticstrength and fatigue strength of the shaft member 1 for the reasondescribed above. To increase strength, the position in which the outerperipheral member 2 is fitted on the shaft member 1 should be as closeto the terminal end as possible and therefore it is preferable that, asshown in FIG. 1, the trough 31 a in the tooth 31 of the shaft member 1and the ridge 32 b of the tooth 32 of the outer peripheral member 2 bein contact with each other in the diametrically increased region S1. Inthis case, as shown in FIG. 3, if an arcuate chamfer (radius r) isprovided on the rise portion 32b1 of the ridge 32 b of the outerperipheral member 2 in contact with the trough 31 a, a further increasein strength due to an increase in the area of the fit region F isobtained.

[0050] It is preferable to subject the shaft member 1 to a surfacehardening treatment, e.g., high frequency hardening. In that case, thehigher the surface hardness, the more desirable, as is apparent fromFIG. 4(A), and it is preferable to aim at the highest hardness for thematerial to be used. As for the hardening depth, it is preferable to aimat 0.5 or thereabouts in terms of the hardening ratio (the depth tillthe effective depth/shaft radius) (see FIG. 4(B)).

[0051] Besides this, shot peening may be applied to the shaft member 1to further increase the fatigue strength or the existing measures forincreased strength may be taken; for example, the diametricallyincreased region S of the trough 31 a may be formed by a large arc asshown in FIG. 8 or the trough 31 a of the diametrically increased regionS may be spherically formed as shown in FIG. 9.

[0052]FIG. 16 shows an example in which, in the tooth 31 of the shaftmember 1, the terminal end (large end) 31a1 of the diametricallyincreased region S1 of the trough 31 a is in contact with the ridge 32 bof the tooth 32 of the outer peripheral member 2, particularly the riseportion 32b1, in taper form, at the terminal end, thus increasing thestrength (static and fatigue strength) of the shaft member 1 in the samemanner as in FIG. 1. FIG. 17 shows an arrangement wherein in the tooth32 of the outer peripheral member 2, the rise portion 32b1 at theterminal end of the ridge 32 b is formed with an arcuate chamfer and thecurvature r of the chamfer is made greater than the curvature R of thediametrically increased region S1 of the trough 31 to bring the riseportion 32b1 into contact with the terminal end 31a1 of the trough 31 ain the same manner as in FIG. 16. In either case, the contact betweenthe rise portion 32b1 and the terminal end 31a1 results in the outerperipheral member 2 being positioned at the axial terminal end, thuspreventing the outer peripheral member 2 and the shoulder 1 b of theshaft member 1 from contacting each other.

[0053] As described above, the outer peripheral member 2 is fixed withrespect to the shaft member 1 by locking the inner diameter end at theinlet by a stop ring 14. In this case, owing to errors in processing orthe like, a clearance 15 could be produced between the inner diameterend at the inlet of the outer peripheral member 2 and the stop ring 14,as shown in FIG. 18, sometimes causing the outer peripheral member 2 torattle axially. Such rattling would make it impossible to hold therelation X=0 in the construction shown in FIGS. 16 and 17, causing avariation in shaft strength.

[0054] To avoid this, it is preferable, as shown in FIGS. 19-22, toprovide restricting means 16 a, 16 b between the shaft member 1 and theouter peripheral member 2 for preventing the outer peripheral member 2from moving toward the other axial end (toward the inlet). Therestricting means 16 a, 16 b may be composed of a forcing means 16 a forforcing the outer peripheral member 2 toward one axial end (terminalend) and a pressing means 16 b for circumferentially pressing the tooth31 of the shaft member 1 and the tooth 32 of the outer peripheral member2 against each other. FIGS. 19-21 show examples of the forcing means 16a; FIG. 19 shows a construction (double clip system) in which two stoprings 14 a, 14 b are disposed in the pressed state, with one stop ring14 a serving to prevent slip-off of the outer peripheral member 2, withthe other stop ring 14 b serving to prevent the shaft member 1 and outerperipheral member 2 from rattling; FIGS. 20 and 21 show a constructionin which the stop ring 14 is replaced by elastic members 17 a, 17 b inthe compressed state (FIG. 20 showing a coil spring system using a coilspring 17 a, FIG. 21 showing a waved washer system using a waved washer17 b), the elastic force thereof being used to apply a preload on theouter peripheral member 2 that is axially directed toward the terminalend. As said pressing means 16 b, there may be thought of a constructionin which as shown in FIG. 22, for example, the tooth 31, 32 of the shaftmember 1 or outer peripheral member 2 is provided with a twist angle θ(which is shown in exaggeration, the figure showing a case where theridge 32 b of the tooth 32 of the outer peripheral member 2 is providedwith a twist angle θ) and the shaft member 1 is force-fitted into theouter peripheral member 2 until X=0.

[0055] The restricting means 16 a, 16 b, as shown in FIGS. 1 and 3, maybe likewise applied when the trough 31 a of the tooth 31 of the shaftmember 1 and the ridge 32 b of the tooth 32 of the outer peripheralmember 2 are contacted with each other in the diametrically increasedregion S1 (in this case, the outer peripheral member 2 and the shoulder1 b of the shaft member 1 may be prevented from contacting each other inthe same manner as in FIGS. 16 and 17).

What is claimed is:
 1. A power transmission mechanism wherein a shaftmember and an outer peripheral member disposed on the outer periphery ofsaid shaft member are connected for mutual torque transmission by a fitbetween the teeth of the shaft member and the teeth of the outerperipheral member, and troughs in the teeth of the shaft member arediametrically increased at least at one axial end, said powertransmission mechanism being characterized in that in the diametricallyincreased regions of said troughs, there are fit regions for the teethof the shaft member and the teeth of the outer peripheral member.
 2. Apower transmission mechanism as set forth in claim 1, wherein said fitportion is located radially inward of the outer periphery level of theshaft member.
 3. A power transmission mechanism as set forth in claim 1or 2, wherein in the diametrically increased region of said trough, thetrough in the tooth of the shaft member and the ridge of the tooth ofthe outer peripheral member are brought into contact with each other. 4.A power transmission mechanism as set forth in claim 3, wherein theridge of the tooth of the outer peripheral member which is in contactwith the trough in the tooth of the shaft member is provided with anarcuate chamfer.
 5. A power transmission mechanism as set forth in claim1, wherein the ridge of the tooth of the outer peripheral member isbrought into contact with the large diameter end of the diametricallyincreased region of said trough.
 6. A power transmission mechanism asset forth in claim 3 or 5, including a restricting means for preventingthe outer peripheral member from moving toward the other axial end.
 7. Apower transmission mechanism as set forth in claim 6, wherein therestricting means is composed of a forcing means for forcing the outerperipheral member toward one axial end.
 8. A power transmissionmechanism as set forth in claim 6, wherein the restricting means iscomposed of a pressing means for circumferentially pressing the tooth ofthe shaft member and the tooth of the outer peripheral member againsteach other.
 9. A power transmission mechanism as set forth in any one ofclaims 1 through 8, wherein the shaft member and the outer peripheralmember are connected together by splines or serrations.